How to Test SMTP AUTH using Telnet

Below are instructions on how to test SMTP AUTH against a mail server using Telnet and entering the commands by hand.

The first thing you need to do is get a base64 encoding of your username and password. There are a couple ways to do this, the example below uses Perl:

1. perl -MMIME::Base64 -e 'print encode_base64("username");'
2. perl -MMIME::Base64 -e 'print encode_base64("password");'

What will be returned from each command is a base64 encoding of the username and password; save these as you will need them later. Now connect to the mail server using Telnet:

1. telnet mailserver.com 25
2. Greet the mail server:

1. EHLO mailserver.com

Tell the server you want to authenticate with it:

1. AUTH LOGIN

The server should have returned 334 VXNlcm5hbWU6; this is a base64 encoded string asking you for your username, paste the base64 encoded username you created earlier, example:

1. dXNlcm5hbWUuY29t

Now the server should have returned 334 UGFzc3dvcmQ6;. Again this is a base64 encoded string now asking for your password, paste the base64 encoded password you created, example:

1. bXlwYXNzd29yZA==

Now you should have received a message telling you that you successfully authenticated. If it failed your user/pass may have been wrong or your mailserver is broken.

Below is a log of a real successful SMTP AUTH connection over Telnet:

1. user@localhost [~]# telnet exampledomain.com 25
2. Trying 1.1.1.1...
3. Connected to exampledomain.com (1.1.1.1).
4. Escape character is '^]'.
5. 220-server1.exampledomain.com ESMTP Exim 4.66 #1 Wed, 09 May 2007 23:55:12 +0200
6. 220-We do not authorize the use of this system to transport unsolicited,
7. 220 and/or bulk e-mail.
8. EHLO exampledomain.com
9. 250-server1.exampledomain.com Hello  [1.1.1.2]
10. 250-SIZE 52428800
11. 250-PIPELINING
12. 250-AUTH PLAIN LOGIN
13. 250-STARTTLS
14. 250 HELP
15. AUTH LOGIN
16. 334 VXNlcm5hbWU6
17. dXNlcm5hbWUuY29t
18. 334 UGFzc3dvcmQ6
19. bXlwYXNzd29yZA==
20. 
    
21. 235 Authentication succeeded

Send a test mail using Telnet

Let us try to send an email to the user now. As the "example.com" domain does not really existing your DNS settings will likely not point to the right server. So we are simulating an SMTP session with the telnet command. Install the telnet package if you haven't already:

$> aptitude install telnet

Then establish a TCP connection to the SMTP port:

$> telnet localhost smtp

The server should reply:

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailtest ESMTP Postfix (Debian/GNU)

Great. Postfix is listening and wants us to speak SMTP. First we need to be friendly:

ehlo example.com

Postfix appreciates our friendliness and tells us which features it provides:

250-my-new-mailserver
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

Hey, Postfix, we have a mail from steve@example.com:

mail from:<steve@example.com>

Looks like Postfix is happy with that because return codes that start with a '2' are good news:

250 2.1.0 Ok

Tell Postfix who the recipient of the mail is:

rcpt to:<john@example.com>

Postfix accepts that:

250 2.1.5 Ok

Then we are ready to send the actual mail:

data

Postfix agrees and tells us we can send the actual mail now and end our input with a dot on an empty line:

354 End data with <CR><LF>.<CR><LF>

Okay, type in the mail:

Hi John,

just wanted to drop you a note.
.

Postfix tells us it has received the mail and queued under a queue ID:

250 2.0.0 Ok: queued as A9D64379C4

Thanks, Postfix, we are done:

quit